The present invention relates to a compressor casing for a gas turbine engine, more particularly such a casing having means to vary the innerdiameter of the casing to maintain a minimum clearance between the casing and a rotor located within the casing.
It is common practice to provide a compressor casing with inner and outer concentrically arranged casings, especially in aircraft ducted-fan gas turbine engines.
Such known casings are illustrated in U.S. Pat. No. 4,543,039 as well as French Patent 2,534,982 which also illustrate means for connecting the inner and outer casing portions.
As is well-known in the art, during the operation of such gas turbine engines, the rotor undergoes radial dimension changes depending upon its rotational speed and its temperature. As the speed of the rotor and its temperature increase, the rotor disk, as well as the rotor blades attached thereto, will expand radially outwardly. As the rotational speed of the rotor decreases and/or its temperature decreases, the rotor wheel and the rotor blades will contract radially inwardly.
In order to maintain the optimum gas turbine engine performance, particularly in regard to maintaining a relatively high efficiency as well as a low specific fuel consumption, it is necessary to maintain a certain minimum radial clearance between the tips of the rotor blades and the inner surface of the compressor casing. Quite obviously, it is also necessary to maintain such clearance to prevent any possibility of contact between the rotor and the casing which may cause catastrophic failure of the engine.
A known solution for adjusting the inner diameter of the compressor casings to maintain this radial clearance is set forth in U.S. Pat. No. 4,714,404 which describes a system having mechanical links connected to a control shaft which is driven by an actuating means to radially displace the inner casing of the gas turbine which is formed in segments.
Another example can be found in U.S. Pat. No. 4,696,619 in which linkrods extend between inner and outer casings and means are provided to ventilate the outer casing.
While these known solutions achieve the desired results, they are complex and increase the weight of the gas turbine engine which adversely effects its aeronautical applications. Furthermore, these known systems often require air to be bled from an upstream portion of the compressor to communicate with the ventilating means, such bleeding often degrading the performance of the engine.
Another solution, described in French Patent 2,640,687 utilizes bleed air from the compressor to adjust the inner radius of the compressor casing. The amount of air removed from the compressor is minimized by supplying it to a bellows-type actuator extending between two inner casing portions. This known system is rather delicate in nature, rendering it ineffective for aeronautical applications.